How can i modify a Displacement texture roughness?
Displacement expects a number, not a color. You can see that because the displacement's input is grey whereas a color-value is marked as yellow in the Node Tree.

So you normally convert the color to a value via a RBG to BW Node. You then can use Math nodes to freely manipulate the value until it fits your needs. In your case I would use a Multiply and then play with the factor a bit until satisfied.



I've used a Voronoi Texture as a stand-in for whatever texture you want to use, so you can just swap that out.

Note: I've heard that this conversion is done automatically when you connect a color-output to an value-input. Apparently this won't lead to any problems, so you can leave out the RGB to BW node and just connect the color output to the Math input (thanks @TARDIS Maker).